Thin-film rechargeable batteries have numerous applications in the field of microelectronics. For example, thin-film batteries may provide active or standby power for microelectronic devices and circuits. Active power source applications of the thin-film battery include, for example, implantable medical devices, remote sensors, wireless sensors, semiconductor diagnostic wafers, automobile tire sensors, miniature transmitters, active radio frequency identification (RFID) tags, smart cards, and MEMS devices. Standby power source applications of thin-film batteries include non-volatile CMOS-SRAM memory products such as memory ships for computers, sensors, and passive RFID tags.
In a battery, a chemical reaction takes place between an anode and cathode by interaction of the anode and cathode through an electrolyte that may be a solid or liquid. Liquid organic electrolytes used in conventional lithium-ion batteries pose safety problems because the electrolytes are flammable and are not tolerant to temperatures above about 130° C. The attractiveness of thin-film batteries over conventional batteries is that the electrolyte is a solid or non-flowable material rather than a liquid. Of the solid electrolytes, thin-film batteries typically employ glassy ceramic electrolytes. Solid electrolytes are desirable in cells or batteries where liquid electrolytes may be undesirable, such as in implantable medical devices. Preferred solid electrolytes include materials that are amorphous solids with high melting temperatures (greater than about 900° C.), electrically insulative and ionically conductive.
One of the challenges for thin film battery manufacturers is to provide a thin film battery having solid electrolytes that will have an extended life. An extended life is particularly difficult to obtain with thin film batteries containing anode materials that are highly reactive with oxygen and/or water or water vapor. Various barrier materials have been applied to thin film batteries to reduce the reactivity of the anode materials toward oxygen and/or water or water vapor. However, such barrier materials have met with limited success.
For example thin film batteries must be sealed or packaged in barrier materials in order to be able to operate in an air environment for a practical length of time. A suitable package must limit the permeation of oxygen and water vapor to such a small level as to allow at least 80% of the battery's capacity to be available after months to years of storage and/or operation. Thin film batteries can be stored in dry environments in which the relative humidity is sufficiently low that water vapor permeation is not a life-limiting factor. However, exposure to air reduces battery life to a few days if oxygen permeation is not restricted to a sufficiently low level by a suitable barrier package. In applications such as automobile tire sensors wherein wireless sensors including a thin film battery power source are imbedded in the sidewalls of the tire, thin film batteries also must be protected from hydrostatic pressure.
A thin film encapsulation process may be a suitable method for hermetically sealing a thin film battery, because the encapsulation layers may be deposited using the same equipment employed in making the batteries. However, silicon, tin, and silicon-tin alloy anodes of thin film lithium-ion batteries may expand uniaxially along the orthogonal direction to the film by over 250% during a charge step. Such expansion strains the protective encapsulation material to the point of fracture allowing oxygen and water vapor to rapidly reach the anode. While a polymer film may be able to accommodate the strain imposed by an expanding anode, a polymer film alone does not provide a sufficient barrier to oxygen and water vapor.
As advances are made in microelectronic devices, new uses for thin-film batteries continue to emerge. Along with the new uses, there is a need for high performance thin-film batteries having improved life. In particular, there is a need for rechargeable thin film batteries that have a life approaching at least five years or longer. Accordingly, there continues to be a need for improved hermetic seals for thin film batteries that enable use of such long life batteries in new applications. A need also exists for reducing the undesirable effects of oxygen and/or water or water vapor that becomes trapped within a sealed thin film package encapsulating a battery. There is also a need for batteries that are able to withstand hydrostatic pressures above atmospheric pressure.
With regard to the above, there is provided in one embodiment a method for improving the useful life of a thin film battery containing a solid electrolyte and an anode that expands on charging and long life batteries made by the method. The method provides a hermetic barrier package for the thin film battery that includes an anode expansion absorbing structure and at least one thin film getter disposed on an interior surface of a chamber defined by the hermetic barrier package. An alternative embodiment of the method includes providing at least one thick film getter instead of or in addition to the at least one thin film getter.
Another embodiment of the disclosure includes a long-life thin film battery package containing a solid electrolyte and an anode that expands on charging. The thin film battery package includes a hermetic barrier, an anode expansion absorbing structure and at least one thin film getter disposed on an interior surface of a chamber defined by the hermetic barrier. An alternative embodiment of the thin film battery package includes at least one thick film getter instead of or in addition to the at least one thin film getter.
Yet another embodiment of the disclosure provides a method of making multiple long-life thin film battery packages on a single substrate. The method includes the step of depositing battery layers including cathodes, electrolytes, and anodes through appropriate masks onto the substrate. A hermetic seal is constructed to substantially complete each of the battery packages. Each hermetic seal has an anode expansion absorbing structure. At least one of the thin film battery packages includes at least one thin film getter, at least one thick film getter, or at least one thin film getter and at least one thick film getter disposed on an inner surface of the volume defined by a hermetic sealing step. The open circuit voltage and resistance of each of the thin film battery packages is determined using a wafer prober in conjunction with a programmable electrometer to identify rejected battery packages. Rejected battery packages are ink marked, and the substrate is diced to provide a plurality of thin film batteries.
In still another embodiment, the disclosure includes a method for making a thin film battery package by attaching the components of a thin film battery to a liquid crystal polymer substrate, encapsulating the thin film battery components with a liquid crystal polymer lid including at least one film getter, and substantially sealing the thin film battery components within the liquid crystal polymer substrate and liquid crystal polymer lid.
An advantage of the disclosed embodiments is that improved hermetic seals for thin film batteries having anodes that greatly expand on charging may be provided. While conventional thin film battery packages containing lithium anodes and other anodes that do not greatly expand on charging may use conventional hermetic seals, thin film battery packages that have anodes that expand over about 200 percent of their height may benefit from the improved hermetic seals and sealing methods provided herein.
An additional advantage of some of the embodiments is the presence of one or more getters (including thin film and/or thick film). The presence of one or more getters acts to minimize the undesirable effects of any oxygen and/or water vapor that remains trapped inside the substantially sealed hermetic barrier package.